The present invention relates generally to steam power generating plant, and more particularly to an apparatus and system for drying desuperheated steam useful in a main steam dump system.
Fossil fuel and nuclear steam power generating plants employ the Rankine cycle to convert steam energy into electric power. In the Rankine cycle, superheated steam is produced in a steam generator or boiler which feeds a turbine coupled to an electric generator that produces electricity. The steam cools and loses its superheat as it passes through the high and low pressure sections of the turbine before being exhausted to a condenser, typically a shell and tube steam surface condenser. Circulating water flows through the tube side which cools and condenses the hot steam flowing on the shell side of the condenser. The liquid condensate is collected and returned to steam generator to continue the cycle.
A steam surface condenser in a combined cycle or power plant requires the condenser to sometimes be operated in bypass mode. Bypass operation can occur during a unit start up or during turbine trips during which time the turbine cannot accept main steam flow from the steam generator. High energy superheated steam generated by the steam generator or boiler bypasses the turbine and is directly dumped into the steam surface condenser.
The HEI (Heat Exchange Institute) recommends pressure and enthalpy ranges for the dumping steam. A desuperheating station comprising a desuperheating pressure reducing valve is typically employed to bring the pressure down under 250 psia and enthalpy under 1225 BTU/lb. prior to entering the condenser. The EPRI (Electric Power Research Institute) guidelines are also widely used industry standards in designing these high energy dissipation devices, which are installed in piping runs called bypass steam headers.
Steam conditioning is critical for safe energy dissipation inside a condenser. Condensers operate at near vacuum conditions (e.g. 1-2″ Hga) at the time bypass mode operation commences. This causes steam to exit at sonic conditions inside the condenser. A small carryover of water droplets that have not had time to evaporate in the surrounding superheated steam can cause significant damage to the condenser internals by wet steam erosion. The effect of wet steam damage has been widely documented.
A typical desuperheating and pressure-reducing station used in steam bypass headers uses spray cooling water such as condensate which is mixed with and desuperheats the steam. Standard design practice is to place the station far enough away from the condenser so that complete evaporation of the water sprayed to accomplish desuperheating has enough time to evaporate in the bypass header piping before reaching the condenser inlet nozzle. Sufficient residence time is required to ensure 100% evaporation of the spray water for minimizing the effects of wet steam erosion. Conversely, if the location of the desuperheating station is too close to the condenser, there may not be enough time to allow for proper mixing and evaporation of the spray water inside the piping before steam exits at high velocity into the neck or dome of the condenser. In such a case, the entrained water droplets can cause significant damage to the condenser internals. Accordingly, the lengthy run of bypass header piping necessary to provide satisfactory residence time for evaporating the entrained water piping can often be difficult to accommodate in the space available within the power plant without interfering with the many other auxiliary systems and equipment used.
An improved approach to handling bypass steam flow to the steam surface condenser is desired.